The continued monitoring of pH characteristics of water-based printing inks has become an important factor when using such inks in printing systems and it is desirable that effective techniques be devised for doing so automatically in a continual, in-line, manner. At the present time, monitoring the pH characteristics of inks in such printing systems is primarily done manually, usually by using a hand-held pH probe having a glass membrane surface which is inserted from time to time into the ink as it flows from an ink pump source to the printer.
Automatic pH probe techniques that have been proposed, in which the probe is merely inserted into the line to project into the flow path, tend to raise a problem in that, when the ink pump source is turned off, the ink flowing through the line in which the probe is located drains back into the pump source leaving the glass membrane surface of the probe exposed to air so that any residual undrained ink still present on portions of the probe surface tends to dry thereon. Over time as such activity occurs, the effectiveness of the probe is reduced or destroyed, by the pressure of such residual, dried ink (the probe is sometimes referred to as having been "poisoned") and the probe must then be replaced Depending on how often the ink flow in the line is shut off, e.g., when the ink being used is temporarily not required or when a different color ink from that currently being used is required in the line so that a new ink source must be attached thereto, probe replacement may be required relatively often thereby raising the material and labor costs of the printing process.
It is desirable to provide an in-line pH probe monitoring device which automatically provides continued pH measurements, so long as ink flows in the line, and in which the need for frequent replacement of the probe is reduced, even when the flow of an ink is temporarily stopped for whatever reason.